1. Field of the Invention
The present invention relates to a displacement detecting encoder and, in particular, to a displacement detecting encoder which is appropriately applicable to an absolute measurement type encoder using a pseudo random code and able to correct errors by using a minimum code without adding a redundant code.
2. Description of the Related Art
There is known a displacement detecting encoder (also referred to as an ABS encoder) for detecting an absolute (ABS) position by using a scale in which a grating corresponding to a pseudo random code as shown in the case of Maximum Length in FIG. 1 is formed as a detection pattern.
In the ABS encoder that uses a pseudo random code, such a problem is posed that dust and the like are attached on a scale to cause an error in detecting the pseudo random code, thereby reducing the reliability of generating ABS position signals.
In order to solve the above-described problem, Japanese Published Unexamined Patent Application No. 2001-296145 (Patent Document 1) has disclosed that detection codes read from a scale containing a redundant pattern are sequentially shifted to calculate absolute position data, and a possible occurrence of a reading error is judged with reference to whether each of the thus shifted and calculated absolute position data is continued.
Japanese Published Unexamined Patent Application No. 2004-117360 (Patent Document 2) has also disclosed that where there is found a decreased difference (corresponding to contrast of transmitted light) in analog scanning signals generated by scanning an interpolatory constituted region due to dirt and the like, such a result is not subjected to calculation.
However, the technology described in Patent Document 1 not only needs to add a redundant pattern but also needs to calculate absolute position data, while shifting detected code, thus resulting in a greater calculation load.
Further, the technology described in Patent Document 2 makes a judgment by referring only to the contrast of transmitted light, thus resulting in a lower accuracy of detecting errors.
Still further, the technologies described in Patent Documents 1 and 2 have a problem of only being able to detect an error but not being able to correct the error.